How do you know that "+" doesn't depend on pi? Oh. Now you see the problem.
Yes, anything _can_ depend on any and all globals, but with proper practices, documentation, code reviews and what have you, one can make simplifying assumptions on what _will_ happen.
Or you can leave out the practices, documentation, and code reviews and use a functional language and get the same result. Yes, those things are useful for other reasons, but the current argument is about avoiding the inherent dangers of global variables.